The present embodiments relate to contactless transmission of electrical signals between two units.
Electrical signals are transmitted between units that are moving relative to one another. In the case of a computed tomography system, for example, the data captured by rotating x-ray detectors is forwarded to a stationary part of the computed tomography system. The stationary part may process the data, slideways and slipcollector rings are used for the transmission. The electrical signal that is supplied to a conductor is diverted by a movable tap. Taps may include contact springs or carbon rods that establish a galvanic contact, alternatively, DE 2845438 describes electrical signals that are transmitted contactlessly using a capacitive or inductive coupling.
A broadband signal transmission between units that are moving relative to one another includes sophisticated high-frequency transmission technology. During transmission, the effect of interference radiation or sensitivity to irradiation should be minimized. In addition, the signal transmission should be low in terms of noise and distortion.
A data source of, for example, a moving unit delivers a discrete symbol sequence including a sequence of binary symbols (e.g., −1 and +1) or a sequence of multilayer symbols (e.g. −7, −5, −3, −1, +1, +3, +5 and +7). The absolute value of the range of the discrete symbol sequence is negligibly low below a limit frequency, so a perfect reconstruction of the symbol sequence in a receiver is also possible even if a weakening distortion of the signal components occurs below the limit frequency during a transmission between the data source and a receiver. Symbol sequences may be generated, for example, using the 8B/10B or 64B/66B encoding used in a plurality of data transmission standards or another method for generating such symbol sequences.
The discrete symbol sequence is converted into an analog signal using a line driver. The analog signal is injected into a line. The signal is terminated at the end with its characteristic wave impedance. The symbols are converted into an analog signal at a fixed data rate. Mathematically, this process can be described as the convolution of an equidistant Dirac pulse sequence whose pulse strengths correspond to the values of the symbol sequence with an analog transmit pulse. The shape of the analog transmit pulse depends on the frequency response of the line driver. A nonlinear pre-distortion, which may be referred to as pre-emphasis, of the analog data signal may be performed in the line driver. The nonlinear pre-distortion results in an unmodulated data signal in the baseband. The spectrum of the analog data signal may have zeros on the line in the case of integer multiples of the symbol rate. In the range below the limit frequency the spectrum is negligibly small.
The line may be, for example, a single line on which the data signal is transmitted in basic mode, or a parallel-routed dual line on which the data signal is transmitted in differential mode.
A stray signal of the line in the local area is contacttessly tapped with a metallic structure, which may be referred to as a coupling element. The metallic structure does not touch the line but is immediately adjacent to the line and can be moved along the line. The shape of the line may be irrelevant. For example, the line may be shaped as a straight line guide for data transmission to translatorily movable units or a circular line guide for data transmission to rotating units.
The weak electrical signal tapped by the coupling element is routed via a line to an amplifying receiving element. Linear, passive filtering may be performed before routing the electrical signal. Linear amplifiers or nonlinear amplifiers, such as limiting amplifiers or comparators, may be used, for example, as the amplifying input stage of the receiving element. The transmitted symbol sequence is then reconstructed with a circuit for reconstructing the symbol timing of the transmission and a sampling of the received data signal.
During the transmission of the electrical signal, decoupling a stray field, if the data rate is greater than approximately ten times the lower limit frequency, may be problematic. Decoupling the stray field leads to interfering reflections at the limits of the coupling element and to interfering propagation delay effects within the coupling element. As a result, the data transmission is prone to errors. Interferences may be avoided if the geometric dimensions of the coupling element are considerably smaller than the smallest wavelength that is to be transmitted.
The signal quality of the digital data transmission may be assessed with an eye diagram from digital data transmission technology. The data stream is subdivided into sections of equal length which are a multiple of the symbol duration in length. The sections are written on top of one another with a persistent oscilloscope or memory oscilloscope. An image in the form of an eye is produced on the screen. If the dimensions of the coupling element are great, a closed eye or a small eye opening is formed in the associated eye diagram, due to reflections. A closed eye or a small eye opening is a sign of a poor digital signal transmission.
For low-frequency signal components, however, reflections and delay effects at the coupling element due to its dimensions are irrelevant. The coupling may be modeled approximately as a discrete capacitance between line and coupling element. If the distance of the coupling element from the line cannot be reduced due to ancillary mechanical conditions, for example, tolerances or insulation gap, this capacitive coupling remains weak. With a low lower limit frequency, the input impedance of the amplifier circuit of the receiving element is highly resistive in order to be able to transmit the low-frequency signal components nonetheless. Otherwise the opening of the eye in the eye diagram is reduced, such that data transmission does not occur.
Characteristic wave impedance matching of the line to the receiver, which is required for a low-reflection transmission, cannot be implemented technically, since high-resistance lines, for example, cannot be produced on printed circuit boards. Accordingly, undesirable reflections result on the line between coupling element and receiving element. At high data rates, the reflections distort the data signal, which prevents an error-free reconstruction of the data in the receiving element.
A small capacitive coupling may not adequately drive parasitic elements of the circuit components that are present at the input of the amplifier circuit of the receiving element. The maximally transmissible bit rate may be limited. Stability problems at the amplifier may occur if the chosen input impedance is too high.
The input impedance of the amplifier circuit of the receiving element may be highly resistive, as a result of which the maximally transmissible bit rate is disadvantageously limited.